The present invention relates to impedance cancellers in general, and in particular to an improved impedance canceller for use with two-wire transmission systems and which has excellent common mode rejection to noise.
Metallic noise across two-wire telephone transmission pairs is undesirable since it affects both the quality of transmission and proper operation of equipment connected with the line. As is well known, to decrease the effect of metallic noise it is desirable to have both longitudinal balance and low impedance to ground in the tip and ring circuits of a telephone pair.
Nulling out equal voltages on the line of a telephone pair is called common mode noise rejection. It is well known, for example, that a well balanced, twisted telephone pair operating metallically provides rejection to noise inductively coupled from power lines. This is accomplished not only by the twisting imparted to the pair, but also by their having balanced longitudinal impedances to ground and equal series impedances.
Although much can be done to the tip and ring leads to enhance common mode rejection to noise, with prior art techniques some longitudinal noise nevertheless remains. In addition, equipment interfacing with the telephone pair can cause longitudinal unbalance and thereby increase noise on the pair. It is therefore very desirable to provide means for interfacing with a telephone pair without degrading the longitudinal balance thereon, while actually enhancing common mode rejection to noise by means of a low impedance balanced path to ground.
Since undesirable impedances often exist in a telephone pair, and since it is usually necessary to have a selected impedance across the pair, it would also be advantageous if the means for interfacing cancelled unwanted impedances from the pair while exhibiting a desired impedance across the pair.
Hybrid circuits are used in long distance telephone transmission facilities to interface between two and four-wire circuits. The four-wire circuit comprises one pair of wires called a receive channel and another pair called a transmit channel, and the conventional two-coil hybrid circuit couples signals from a distant subscriber on the receive channel with the two-wire circuit leading to a local subscriber, and couples signals from the local subscriber on the two-wire circuit with the transmit channel leading to the distant subscriber. It would therefore also be advantageous to provide an improved and accurate electronic hybrid circuit which provides common mode rejection to metallic noise on telephone pairs.
It is well known that current sources provide exceptional accuracy and would be very desirable circuit mediums or interface units for telephone applications, except that conventional current sources exhibit no common mode rejection to noise, but instead introduce longitudinal unbalance onto a telephone pair. Such undesirable characteristics of conventional current sources severely restrict their use in telephony, despite the fact that current sources otherwise exhibit other properties which would enhance their use.